The present invention generally relates to apparatus operably positionable in the wellbore of a subterranean well and, in a preferred embodiment thereof, more particularly provides specially designed latching apparatus and associated methods for operatively coupling a firing head structure to a perforating gun.
In subterranean wells, such as oil and gas wells, it is common practice to facilitate the flow of production fluid by perforating a fluid bearing subterranean formation using a device commonly referred to as a perforating gun which is lowered into the wellbore to the depth of the formation and then detonated to form perforations in the formation surrounding the gun. A firing head assembly is operatively coupled to the gun and detonated to fire the gun. While the firing head assembly may be coupled to the perforating gun before the gun is lowered into the wellbore, it is often preferred, for safety and other reasons, to couple the firing head to the gun after the gun is positioned downhole in the wellbore.
For the lowered gun to function, it must be properly coupled to the subsequently lowered firing head. This downhole coupling, or xe2x80x9clatchingxe2x80x9d, of the firing head to the associated perforating gun has heretofore been subject to several problems, limitations and disadvantages. For example, one previously proposed firing head/perforating gun latching system utilizes flexible collet fingers on the firing head that are designed to be outwardly deflected over an upper end of an associated stinger portion of the perforating gun, and then snap into a circumferential groove in the stinger to operatively latch the firing head to the perforating gun.
The collet fingers, as they approach the stinger, pass though a centering restriction in the tubing on which the perforating gun has been previously lowered into the wellbore, and through which the firing head passes on its way to the perforating gun. This centering restriction is designed to laterally align the collet fingers with the upper end of the stinger, but can easily be struck by and inwardly bend one or more of the collet fingers, thereby preventing the proper latching between the firing head and the perforating gun. This same undesirable bending of the collet fingers could also result from the collet structure striking some other obstruction or irregularity in the tubing as the collet structure passes through it in a downhole direction toward the previously lowered perforating gun.
A potential solution to this downhole firing head/perforating gun latching problem is simply to attach the firing head to the perforating gun at the surface, and then lower the coupled firing head and perforating head into the wellbore together. However, as previously mentioned, in many instances this is considered undesirable from safety and other standpoints. Additionally, if for some reason the firing head malfunctions, both the firing head and the perforating gun must be pulled from the wellbore, as opposed to simply pulling and replacing the malfunctioning firing head.
As can readily be seen from the foregoing, a need exists for improved apparatus and associated methods for effecting the downhole latching of a firing head to a previously lowered perforating gun. It is to this need that the present invention is directed.
In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, perforating apparatus is provided which is operatively positionable in a subterranean wellbore and includes a firing head and an associated perforating gun. Cooperatively engageable first and second latching structures are preferably of fixed geometry configurations, are respectively carried by the firing head and the perforating gun, and are operative to couple them, while in the wellbore, in a manner such that subsequent operation of the firing head responsively fires the perforating gun. According to an aspect of the present invention, a portion of one of the first and second latching structures is shearable in a manner permitting the firing head, after being coupled to the perforating gun in the wellbore, to be disengaged from the perforating gun and retrieved from the wellbore.
In an illustrated embodiment of the present invention, the first latching structure is representatively a tubular latch collar portion of the firing head and has a circumferentially spaced plurality of shearable lugs extending radially inwardly into its interior. The second latching structure is representatively a stinger portion of the perforating gun, is telescopingly receivable in the latch collar, and has a circumferentially spaced plurality of J-slot recesses formed on an exterior sidewall portion thereof. As the latch collar is telescoped onto the stinger, the shearable studs enter the stinger J-slots to couple the firing head to the perforating gun.
In accordance with a method of the invention, the perforating gun is lowered into the wellbore to a predetermined depth therein and held at such predetermined depth. The firing head is then lowered, on a suitable lowering structure such as a slickline, into the wellbore until the latching portions of the firing head and perforating gun are interengaged. The slickline is then pulled up to verify, via a sensed increase in its tension, that the lowered firing head has been properly latched to the previously lowered perforating gun. After proper firing head/perforating gun latching has been verified the firing head is appropriately actuated to responsively fire the perforating gun. Subsequent to the firing of the perforating gun, the spent firing head is pulled up with sufficient force to shear the shearable portion of the latching structure, for example the shearable latch collar studs, thereby releasing the firing head from the perforating gun and permitting the unlatched firing head to be pulled out of the wellbore.
According to another aspect of the present invention, cooperative auxiliary connecting structures are provided on the interengageable latching portions of the firing head and perforating gun which permit them to be fixedly secured to one another in a manner permitting the perforating gun and firing head to be simultaneously lowered into the wellbore in an operatively connected state instead of being sequentially lowered into the wellbore and operatively latched together therein. Representatively these cooperative auxiliary connecting structures include a circumferentially spaced plurality of openings formed in the sidewall of the latch collar and alignable with side surface depressions in the stinger, and connecting members extendable through the collar openings into the stinger depressions to longitudinally and rotationally lock the collar onto the stinger.